This invention relates to sensors for detecting residual combustibles in exhaust gas, especially internal combustion engine exhaust gas. More particularly, it relates to a thermoelectric device that has a high output, yet which is durable in the highly corrosive and abrasive automobile engine exhaust gas environment.
Thermocouple systems are known for sensing residual combustibles in an exhaust gas. They sense heat generated by oxidizing residual combustibles in an exhaust gas. This heat is proportional, within certain limits, to the concentration of the residual combustibles in the exhaust gas. One junction of a thermocouple can be coated with a catalyst material, while the other junction is left uncoated, to serve as a reference. The residual combustibles oxidize on the catalyst, making the coated junction hotter than the uncoated one. The thermocouple detects the resulting difference in temperature between the two junctions. Such devices can be useful, for example, in monitoring efficiency of an automobile engine and/or its catalytic converter.
Commercially available thermocouple exhaust gas sensors have had their disadvantages. Most devices have not provided an output signal large enough to be easily differentiated from noise that is common to automotive applications. Further, they generally have not been durable in the corrosive and abrasive exhaust gas environment and have been relatively complex to manufacture. Moreover, the design of these sensors introduced inaccuracies in their output. Many also had unwanted thermal gradients between the two thermocouple junctions, caused for example by the decreasing temperature of the exhaust gas as it flows from the engine. Accordingly, such sensors have not been readily accepted in applications which require a reliable, high output device.
U.S. Pat. No. 3,913,058 Micheli et al discloses a significant improvement in such sensors. Chromel wires forming two separate thermocouple junctions are embedded in the ends of a doped titanium dioxide U-shaped body. An oxidation catalyst in a recess near one of the thermocouple junctions provides the junction temperature differential. This latter device is extremely durable and provides a high output.
We have now found an even higher output device that appears to be as durable. It does not require chromel wires or a large quantity of titanium dioxide. Moreover, our higher output device can be made in a form analogous to a spark plug, facilitating its manufacture in high volume commercial production.